The edge table defines these relations between resources. It does
this by adding a directed edge from one rsc record (subject)
to another (object). It also adds a reference to the
predicate: the label of the edge.

In the admin, edges are represented in the “Page connections” sidebar
panel, of the edit page of the subject: the resource where the edges
originate. By convention, edges are said to belong to their subject.
This is to simplify the access control: if you are allowed to edit the
resource, you’re also allowed to edit its outgoing edges (“Page
connections” in the admin), creating connections to other resources.

Just like categories, these predicates are themselves also resources:
allowing you to specify metadata, give them a meaningful title, et
cetera.

Each predicate has a list of valid subject categories and valid object
categories (stored in the predicate_category table). This is used
to filter the list of predicates in the admin edit page, and also to
filter the list of found potential objects when making a
connection. On their edit page in the admin interface, you can edit
the list of valid subject and object categories for a predicate.